1. Field of the Invention
This invention relates generally to microelectronics and more specifically to metal-semiconductor, metal-metal, and semiconductor-semiconductor junctions in nanotubes.
2. Description of Related Art
The electronic structure of carbon nanotubes is governed in part by the geometrical structure of the tube (R. Saito et al., Appl Phys Lett, 60:2204, 1992; and N. Hamada et al., Phys Rev Lett. 68:1579, 1992).
A nanotube index (n,m) has been developed to describe the nanotube structure. It is described in detail by C. T. White et al. xe2x80x9cPredicting Properties of Fullerenes and their Derivativesxe2x80x9d, Chapter 6, page 159 and following, in Buckminsterfullerenes, W. E. Billups, and M. A. Ciufolini, ed. (NY: VCH Publishers, 1993). Carbon nanotubes can be understood by thinking of them as a graphite sheet, in which the carbon atoms are arranged in a honeycomb lattice of hexagonal rings. The sheet is rolled up and spliced together to form a tube. That is, the tube is a conformal mapping of the two dimensional sheet onto the surface of a cylinder. The two-dimensional lattice sheet can be rolled many different ways to form a tube. The nanotube index describes how a sheet is rolled into the tube. A special circumference vector is related to the number of adjacent hexagonal carbon rings that are traversed when tracing the tube circumference once, and the amount the lattice is skewed when it is rolled. The lattice vector, R, is made up of two component vectors, R1 and R2, where R=nR1+mR2 with n and m integers or zero.
Electrical properties of nanotubes are associated with the nanotube index as follows:
1. Carbon nanotubes having a nanotube index in which n=m, for example, (5,5) or (9,9), are metals. These nanotubes conduct electrical current.
2. Carbon nanotubes, characterized by an index of (n,m) where nxe2x88x92m is a nonzero multiple of three, for example, (6,3) or (12,0). These nanotubes have a band-gap that is typically less than 0.1 eV, characteristic of semiconductors or semimetals. The size of the band-gap is inversely proportional to the tube radii. However, if the radius is very small as described by X. Blase et al., Phys Rev. Lett. 72: 1878, 1994, in which case, the large curvature further modifies the nanotube electrical properties.
3. Carbon nanotubes having nanotube indices different from those described above, for example, (7,0) or (13,4), are semiconductors that have a band-gap size up to approximately 1 eV. The size of the band-gap is inversely proportional to the tube radius (J. W. Mintmire et al., Mater. Res. Soc. Sym. Proc. 247:339, 1992).
Typically, a useful junction of two semiconductors requires that the two semiconductors have band-gaps whose difference exceeds the electron thermal energy.
T. W. Ebbesen and T. Takada (Carbon, 33: 973, 1995) have noted that the index of a tube can be changed by introducing topological defects into the hexagonal bond network of carbon. Theoretically nanotubes can be constructed to have a particular index and exhibit particular electrical properties. To result in a useful structure, the defects may not induce a net curvature which might cause the tube either to flare or to close, and minimal local curvature surrounding the induced defect is desirable to minimize any defect energy. Since defects can change the index in a nanotube, they are considered to be the cause of some flaring or closing of the tube structure as a result of the introduced defect in the hexagonal carbon lattice.
While some researchers have described nanotubes having any of a single pair of indices, (n,m), no-one has described how a continuous tube could be formed having separate sections that would be characterized by different indices, (n1, m1) and (n2, m2).
No-one has discovered a means to change the nanotube index within a continuous carbon nanotube without causing the net curvature to change so that one end closes or flares. Another way to say this is that no one has found a way to join two carbon nanotubes having different nanotube indices to form a continuous tube.
It would be highly desirable and useful to alter the index within a continuous tube in a manner that that does not change the net curvature of the tube but does alter the nanotube""s electrical properties on either side of specific point so that adjoining sections of the nanotube are semiconducting, non-conducting, or conducting as needed for semiconductor devices and circuits. Another way of stating this is that it would be very useful to be able to join two nanotubes having different indices through a junction.
It is an object of the invention to design carbon nanotubes containing adjacent sections of differing electrical properties. These nanotubes can be constructed from combinations of carbon, boron, nitrogen and other elements. It is a further object of the invention to design carbon nanotubes in which the nanotube index is different on either side of a junction point in the tube so that the electrical properties on either side of the junction vary in a useful fashion. For example, a carbon nanotube may be electrically conducting on one side of a junction and semiconducting on the other side. An example of a semiconductor-metal junction is a Schottky barrier. Alternatively, the carbon nanotube may exhibit different semiconductor properties on either side of the junction. A junction that joins materials having different semiconducting properties on either side of the junction is sometimes referred to as a heterojunction. Nanotubes containing heterojunctions, Schottky barriers, and metal-metal junctions are useful for microcircuitry.
The present invention comprises a new nanoscale metal-semiconductor, semiconductor-semiconductor, or metal-metal junction, made by introducing topological defects in the essentially hexagonal carbon atom structure of a carbon nanotube. The defects change the carbon lattice arrangement such that the nanotube index on one side of the defect is characteristic of metal properties and the index on the other side of the defect is characteristic of semiconductor properties. A junction is thus formed at the site of the defect. The inventive nanotube comprises carbon bonded in an essentially hexagonal array, wherein the array contains some non-hexagonal carbon rings. The non-hexagonal rings comprise a defect that changes the index and forms a junction point in the nanotube. The point where the index changes, that is the point where the defect exists, forms a junction between carbon nanotubes having different electrical conduction properties. The invention comprises essentially carbon nanotubes having metal-semiconductor junctions, semiconductor-semiconductor junctions, and metal-metal junctions. Similar junction effects can be achieved in nanotubes with local chemical additions, subtractions, or substitutions.
FIG. 1: schematically shows the atomic structure of a carbon nanotube in which a heptagon-pentagon carbon-ring structure has been introduced.
FIG. 2: shows a transmission electron micrograph of a carbon nanotube with an extended pentagon-heptagon defect.
FIG. 3: shows the Density of States (DOS) verses Energy for the semiconducting section of a nanotube having nanotube index (8,0), and adjoining a (7,1) section.
FIG. 4: shows the Density of States verses Energy for the conducting, metallic, section of a nanotube having nanotube index (7,1), and adjoining a (8,0) section.
FIG. 5: shows the Density of States verses Energy for the semiconducting section of a nanotube having nanotube index (8,0), and adjoining a (5,3) section.
FIG. 6: shows the Density of States verses Energy for the semiconducting section of a nanotube having nanotube index (5,3), and adjoining a (8,0) section.
FIG. 7a: shows a side view of the structure of a carbon nanotube having two electrically conducting sections on either side of a junction in the carbon lattice, wherein the electron resonances in each section are different.
FIG. 7b: shows a view down the long axis of the same tube, looking toward the junction.
FIG. 8: shows a nanotube having oppositely-oriented pentagon-heptagon pairs.